An organic LED element is one in which an organic layer is put between electrodes, and a voltage is applied between the electrodes to inject holes and electrons, which are allowed to be recombined in the organic layer, thereby extracting light that a light-emitting molecule emits in the course of transition from an excited state to a ground state, and has been used for display, backlight and lighting applications.
The refractive index of the organic layer is from about 1.8 to about 2.1 at 430 nm. On the other hand, the refractive index, for example, at the time when ITO (indium tin oxide) is used as a translucent electrode layer is generally from about 1.9 to about 2.1, although it varies depending on the ITO film-forming conditions or composition (Sn—In ratio). Like this, the organic layer and the translucent electrode layer are close to each other in refractive index, so that emitted light reaches an interface between the translucent electrode layer and a translucent substrate without totally reflecting between the organic layer and the translucent electrode layer. A glass or resin substrate is usually used as the translucent substrate, and the refractive index thereof is from about 1.5 to about 1.6, which is lower in the refractive index than the organic layer or the translucent electrode layer. Considering Snell's law, light which tries to enter the glass substrate at a shallow angle is reflected by total reflection in an organic layer direction, and reflected again at a reflective electrode to reach the interface of the glass substrate again. At this time, the incident angle to the glass substrate does not vary, so that reflection is repeated in the organic layer and the translucent electrode layer to fail to extract the light from the glass substrate to the outside. According to an approximate estimate, it is known that about 60% of the emitted light cannot be extracted by this mode (organic layer-translucent electrode layer propagation mode). The same also occurs at an interface between the substrate and the air, whereby about 20% of the emitted light propagates in the glass and fails to be extracted (substrate propagation mode). Accordingly, the amount of the light which can be extracted to the outside of the organic LED element is less than 20% of the emitted light in the present circumstances.
On the other hand, Patent Document 1 has proposed a structure having on one side of a substrate a light-scattering layer as a semi-translucent material layer (in paragraphs 0039 to 0040). Further (in paragraph 0070), the document has proposed e.g. a structure having a light-scattering region provided between a substrate and an organic LED element by placing glass particles in an aggregate pattern on the substrate surface and sticking them to the substrate surface by the use of an acrylic adhesive.
By providing such a light-scattering region, the reflective electrode surface is not visually recognized as a mirror-like surface, and the outward appearance thereof is improved.
On the other hand, when the scattering ability is lowered or the light-scattering region is not provided, the reflective electrode is visually recognized as a mirror surface, and probably has undesirable outward appearance.
Alternatively, there is a case where a glass substrate formed by the float process is used in an organic LED element. In this case, the substrate tends to suffer waviness resulting from waves caused in a fused metal bath, contact with rolls for conveyance of a glass ribbon, variation of temperature in a cooling process and so on. Therefore, it has been known to mechanically polishing the substrate surface for the purpose of achieving flatness. However, the polished glass substrate suffers fine polishing scratches formed on the surface, and these scratches become a cause of a short-circuit occurring between an anode and a cathode.
Therefore the organic LED element designed to form an electrode on an surface having waviness has been proposed (Patent Document 2).